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Abstract

Capillaries present a promising structure for microfluidic refractive index sensors. We demonstrate a capillary-type fluorescent core microcavity sensor based on whispering gallery mode (WGM) resonances. The device consists of a microcapillary having a layer of fluorescent silicon quantum dots (QDs) coated on the channel surface. The high effective index of the QD layer confines the electric field near the capillary channel and causes the development of WGM resonances in the fluorescence spectrum. Solutions consisting of sucrose dissolved in water were pumped through the capillary while the fluorescence WGMs were measured with a spectrometer. The device showed a refractometric sensitivity of 9.8 nm/RIU (up to 13.8 nm/RIU for higher solution refractive index) and a maximum detection limit of ~7.2 x 10−3 RIU. Modeling the field inside the capillary structure, which is analogous to a layered hollow ring resonator, shows that sensitivities as high as 100 nm/RIU and detection limits as low as ~10−5 RIU may be achievable by optimizing the QD film thickness.

Figures (9)

Schematic of the fluorescent capillary structure. The WGMs are confined in the QD film coated on the inner capillary surface. The fields associated with the TEz polarization are shown with colored arrows. The polarization is defined with respect to the plane of propagation of the WGMs.

(a) Scanning electron micrograph showing part of a quantum dot film extruding from the cleaved end of an FCM channel. (b) TEM micrograph from a flat film, showing several QDs embedded in a glassy matrix (highlighted by the white ellipses). Electron diffraction (not shown) confirmed the presence of randomly-oriented Si-QDs.

PL spectra for a) 100 μm ID and b) 25 μm ID capillaries indicating TMz and TEz WGMs, separated using a linear polarizer. The TEz polarized modes (electric field parallel to the capillary axis) are more intense than for the TMz case, and were therefore used for the refractometric measurements. Data offset for clarity.

Shifts in WGM fluorescence for different sucrose solutions in the channel of (a) a 25-μm-ID Type-I capillary; (b) same as in (a) but with a 405-nm LED pump; and (c) a 100 μm-ID Type-II capillary. Insets show the WGM wavelength shifts.

FDFD-calculated WGM resonance shifts for Type-I (solid) and Type-II (dashed) FCMs, as a function of QD layer thickness. Inset Refractometric sensitivity of Type-I (solid) and Type-II (dashed) capillaries at n1 = 1.375 (solid vertical line in main figure). Film thickness influences the refractometric sensitivity of the device, sharply increasing for “thin” films. For a given film thickness, the larger diameter capillary shows higher refractometric sensitivity.

A single mode from a Type-I capillary, with a double-Lorentzian fit. Data was fit in frequency space then translated to wavelength for visualization. The inset shows the experimental and calculated resonance shifts. The solid line corresponds to the shift obtained from FDFD computations for the l = 160 mode in a Type-I capillary with a 525-nm-thick film. The resonance shift of this peak is approximately quadratic with respect to the refractive index of the analyte, n1, as shown by the dashed curve, which included the n1 = 1 data point in the fit. The maximum refractometric sensitivity, given by the slope of this curve at n1 = 1.45, is 13.8 nm/RIU (whereas, the linear sensitivity over the more limited range shown was 9.8 nm/RIU).

Shift in peak resonance wavelength of a WGM in (a) a Type-II capillary; and, (b) a Type-II capillary, as a function of incident laser power. The slope of the least-squares linear fit gave a thermal shift of 0.4 pm/mW of laser power for Type-I FCMs and 1.4 pm/mW for Type-II. Some of the jitter in the data is due to random errors in the measurement or the peak fitting routine, as discussed previously. All shifts were measured with respect to the first data point (which has a zero shift, by definition).